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Dogs come in countless shapes and sizes and exhibit more diversity than any other land mammal on earth, a fact that makes them especially appealing to geneticists. In the last decade, scientists working at more than 100 laboratories worldwide have made significant progress in painting a detailed picture of the complex relationships between physical traits, behavior and disease in purebred dogs, and the ways genes contribute to the striking differences seen across breeds.

Searching for related traits isn’t a new idea. Literature indicates that as far back as the fourth century BC, people were looking for something physical, like tail carriage or ear length, to predict something intangible — courage or sagacity, for example. The Greek philosopher Xenophon, describing the interests and values of sporting men, said that tan-colored dogs with black muzzles were esteemed as the best hunters. Early 16th-century New World explorers differentiated Indian dogs from wolves by the curve of their tails and their vocalizations. The 1785 edition of the Sportsman’s Dictionary advised serious huntsman: “Coal black dogs prove incomparable hounds.” On the other hand, the future was dubious at best for those dogs unlucky enough to be covered with spots: “Forego white hounds with black spots as they are never the best hare hunters.”

New genetic research indicates that yesteryear’s cynologists were on the right trail, but barking up the wrong trees. Dog breeds are not mix-andmatch combinations of thousands of small parts; rather, they are more like combinations of genes prepackaged in bundles and shuffled around.

So, yes: traits are linked to one another, but with caveats. As scientists and statisticians note, correlation is not causation — just because two phenomena are related, it doesn’t necessarily mean one causes the other. Sometimes, it’s merely coincidence. The rooster crows when the sun comes up, but contrary to what the big chicken thinks, he isn’t the reason it rises.

Molecular Conversations
Genetic interactions are less like a game of marbles, in which one gene strikes another and something happens, and more like a Rube Goldberg contraption: something seemingly simple is actually the result of a long list of intricate, complicated and extraneous events. Of the thousands of genes in the genome (dogs have about 19,000, compared to humans’ approximately 23,000), an inordinate number are involved in communicating with one another, sending information that either activates or represses a “dimmer switch” in other genes. Although numerous genes act alone, thousands play multiple roles during development, telling the cells of an embryo what kind of living creature it is to become.

This molecular maneuvering isn’t readily visible in most mammals. For instance, a human with a genetic variant that codes for extra height may grow to be 10 percent taller than the population average. But dogs are an entirely different story. The same type of genetic code in Canis lupus familiaris may result in a dog that’s 40 percent taller than the population norm.

Coded for Variety
Mapping a genome is like revealing the image on the front of the jigsaw-puzzle box. You know it’s a picture of a Boxer, but you don’t know how all the interlocking pieces fit together to make the picture. Some solutions may be fairly clear, but, for the most part, our understanding of the way genes function is still based on a lot of guesswork.

In simple terms, geneticists use mega-computers to compare and contrast DNA patterns, looking for discrepancies. Patterns with omissions, transpositions or substitutions provide more precise information to help scientists assign meaning to genomic variations. They then match the anomaly in the DNA to the trait of interest.

This is how researchers identified genetic deviations that account for breed differences. In 2004, Heidi Parker, Elaine Ostrander and their colleagues at the Washington-based Fred Hutchinson Cancer Research Center published data indicating that as much as 30 percent of the dog’s genetic material accounts for breed variation (Parker et al. 2004). Whether it codes for breed-specific size, shape, behavior or disease, or the 57 parts that make up the Heinz mutt, the metaphorical genetic needle hides out in one-third of the genomic haystack.

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Jane Brackman, PhD, is an authority on the cultural history of canine domestication and the author of two books on pets in 19th-century America. See her new pup, Barkley, and watch him grow on her blog.